The present invention relates to ion generators, and more particularly to ion generators employed for electrostatic imaging.
A wide variety of techniques are commonly used to generate ions for electrostatic imaging. Conventional approaches include air gap breakdown, corona discharges, spark discharges, and others. The use of air gap breakdown requires close control of gap spacing, and typically results in non-uniform latent charge images. Corona discharges, which are widely favored in electrostatic copiers, provide limited currents and entail considerable maintenance efforts. Electrical spark discharge methods are unsuitable for applications requiring uniform ion currents. Other methods suffer comparable difficulties.
Apparatus and methods for generating ions representing a considerable advance over the above techniques are disclosed in commonly assigned U.S. Pat. No. 4,155,093, issued May 15, 1979. The ion generator of this invention, shown in one embodiment at 10 in FIG. 1, includes two conducting electrodes 12 and 13 separated by a solid insulator 11. When a high frequency electric field is applied between these electrodes by source 14, a pool of negative and positive ions is generated in the area of proximity of the edge of electrode 13 and the surface of dielectric 11. Thus in FIG. 1, an air gap breakdown occurs relative to a region 11-r of dielectric 11, creating an ion pool in hole 13-h, which is formed in electrode 13. This air breakdown is of the "glow discharge" type, characterized by a faint blue glow in the discharge region, at an inception voltage of around 350-400 volts.
These ions may be used, for example, to create an electrostatic latent image on a dielectric member 100 with a conducting backing layer 105. When a switch 18 is switched to position X and is grounded as shown, the electrode 13 is also at ground potential and little or no electric field is present in the region between the ion generator 10 and the dielectric member 100. However, when switch 18 is switched to position Y, the potential of the source 17 is applied to the electrode 13. This provides an electric field between the ion reservoir 11-r and the backing electrode 16. Ions of a given polarity (in the generator of FIG. 1, negative ions) are extracted from the air gap breakdown region and charge the surface of the dielectric member 100. The charge formed on dielectric 100 is seen to increase generally in proportion to the number of excitation cycles of drive potential 14. Because it is necessary in order to form an electrostatic image on dielectric 100 to have a coincident drive voltage 14 and extraction voltage 17, this device is amenable to multiplexing.
One advantageous use of the ion generator disclosed in the above patent is for the formation of electrostatic images such as for high speed electrographic printing. When employed for this purpose, the apparatus of U.S. Pat. No. 4,155,093 encounters certain difficulties discussed in the Background of the Invention of the commonly assigned improvement patent, U.S. Pat. No. 4,160,257. With reference to the prior art sectional view of FIG. 2, the ion generator 20 includes in addition to the above disclosed elements an apertured screen electrode 21, which is separated from the control electrode 13 and solid dielectric member 11 by a dielectric spacer 23. This additional electrode was found necessary to cure the problem of accidental erasure of a latent electrostatic image previously formed on the dielectric surface 100. This would occur in the apparatus of FIG. 1 if a high voltage alternating potential were imposed between the control and driver electrodes, without any extraction potential applied to the control electrode 13. In this instance, any previously formed charge image on the dielectric surface 100 would create an electrostatic extraction field tending to attract of ions of opposite polarity from the control aperture 13-h, thereby partially or completely erasing the electrostatic image. As discussed in detail in U.S. Pat. No. 4,160,257, the inclusion of screen electrode 21 has been found to prevent such accidental image erasure by imposing a screen potential 28 between the screen electrode 21 and counterelectrode 105 of the same polarity as control potential 17.
The significant advantages provided by the three electrode design of U.S. Pat. No. 4,160,257 have been found to be somewhat offset by certain disadvantages of the screen electrode. Perhaps most significantly, the screen electrode tends to attract a significant percentage of the ions emerging from control aperture 13-h, thereby reducing the ion output current of ion generator 20. In some cases, the screen electrode has been found to attract as much as 95 percent of these ions. The reduction in ion output efficiency attributable to this screen transmission loss necessitates the use of significantly higher driving potentials to achieve a desired output current level. This increase in driving potentials in turn incurs other disadvantages, such as an increase in the unavoidable chemical byproducts of the ion generation process, and an aggravation of the voltage stress between adjacent drive electrodes 12 in a multielectrode ion generator. Additionally, the screen electrode 21 complicates the design of ion generator 20, which for example increases the difficulty of cleaning this device.
Accordingly, it is a principal object of the invention to provide an improved ion generator for the formation of electrostatic images. A related object of the invention is to provide an ion generator which achieves the advantages of the device disclosed in U.S. Pat. No. 4,160,256, while avoiding many of the disadvantages of this design.
Another object of the invention is to reduce the power requirements of an ion generator of this type, while maintaining acceptable ion output current levels. A related object is the avoidance of screen transmission losses characteristic of the '256 three electrode device.
A further object of the invention is to avoid undesirable phenomena associated with high driving potentials. A specific object is the reduction of environmental byproducts of the ion generation process. Another object is to reduce voltage stresses by the adjacent driver electrodes, thereby reducing the risk of arcing.
Yet another object of the invention is simplicity of construction of an ion generator. Related objects include facilitating the fabrication of such devices, and reducing maintainance requirements.